Genes associated with specific autosomal recessive syndromes may predispose the heterozygous carrier to cancer and leukemia. Because heterozygotes for these genes are relatively comm n in the general population, some genes identified in this way are likely to be among the important human neoplasia-predisposing genes. Fanconi anemia (FA) and ataxia-telangiectasia are two autosomal recessive syndromes associated with cancer and leukemia in heterozygotes and homozygotes. A systematic procedure has been developed for analyzing the defect determined by the FA gene, based on the observation that several abnormalities of FA fibroblast cultures are corrected by co-cultivation with normal cells. This procedure may be used to analyze the basic metabolic abnormality of several important neoplasia-predisposing genes.